(1) Field of the Invention
This invention relates to radiation detectors of the direct conversion type including a radiation sensitive semiconductor for generating electric charges upon incidence of radiation, for use in the medical, industrial, nuclear and other fields.
(2) Description of the Related Art
An indirect conversion type radiation detector first converts radiation (e.g. X rays) into light, and then converts the light into electric signals by photoelectric conversion. As distinct from the indirect conversion type, a direct conversion type radiation detector converts incident radiation (e.g. X rays) directly into electric signals by a radiation sensitive semiconductor.
As shown in FIG. 1, a direct conversion type radiation detector includes an active matrix substrate 51, a radiation sensitive semiconductor 52 and a common electrode 53 for bias voltage application. A lead wire 54 for supplying a bias voltage is connected to the surface of the common electrode 53. Numerous collecting electrodes (not shown) are formed on the surface of the active matrix substrate 51, in a two-dimensional matrix arrangement set within a radiation detection effective area SA. An electric circuit (not shown) is arranged on the surface of the active matrix substrate 51 for storing and reading electric charges collected by the respective collecting electrodes upon incidence of radiation. The radiation sensitive semiconductor 52 is laid on the surface of the active matrix substrate 51 where the collecting electrodes are formed, to generate charges upon incidence of the radiation. The common electrode 53 for bias voltage application is formed two-dimensionally on the front surface of the radiation sensitive semiconductor 52.
In time of radiation detection by the direct conversion type radiation detector, a bias voltage from a bias voltage source is applied to the common electrode 53 for bias voltage application via the lead wire 54 for bias voltage supply. With the bias voltage applied, electric charges are generated by the radiation sensitive semiconductor 52 upon incidence of the radiation. The electric charges generated by the semiconductor 52 are collected by the collecting electrodes. The electric charges collected by the collecting electrodes are fetched as radiation detection signals from the respective collecting electrodes by the storage and reading electric circuit including capacitors, switching elements and electric wires.
That is, in the direct conversion type radiation detector, each of the collecting electrodes in the two-dimensional matrix arrangement acts as an electrode corresponding to each pixel in a radiographic image (pixel electrode). Radiation detection signals obtained can be used to create a radiographic image according to a two-dimensional intensity distribution of the radiation projected to the radiation detection effective area SA.
However, the conventional radiation detector has a problem that performance lowers as a result of the lead wire 54 for bias voltage supply being connected to the common electrode 53 for bias voltage application.
Since a hard metal wire such as copper wire is used for the lead wire 54 for bias voltage supply, when the lead wire 54 is connected to the common electrode 53, damage is done to the radiation sensitive semiconductor 52, leading to a lowering of performance such as a voltage resisting defect. Particularly where the semiconductor 52 is amorphous selenium or a non-selenic polycrystalline semiconductor such as CdTe, CdZnTe, PbI2, HgI2 or TlBr, the radiation sensitive semiconductor 52 of large area and thickness may easily be formed by vacuum deposition, but such amorphous selenium and non-selenic polycrystalline semiconductor are relatively soft and vulnerable to scratch. Even where a carrier selective intermediate layer that demonstrates a dark current reducing effect is disposed between the radiation sensitive semiconductor 52 and common electrode 53, the carrier selective intermediate layer is far thinner than the semiconductor 52, and therefore a lowering of performance such as a voltage resisting defect will occur with the carrier selective intermediate layer and semiconductor 52 when the lead wire 54 is connected to the common electrode 53.